The proposed experiments examine the suprathreshold responding of normal and abnormal visual systems. The main psychophysical procedure employed is one that involves the detection of a brief light in the presence of second brief light of variable intensity. These detection data can be quantitatively related to models of the response-intensity function of the visual system. Most of our knowledge about the human visual system is based on measurements of threshold; much less is known about the visual system's response to light above threshold. Our overall objective is to increase our understanding of suprathreshold responding and to apply this knowledge to various clinical and non-clinical problems. Among the problems considered in this proposal are the effects of adaptation, the role of chromatic mechanisms, the differences among retinal positions, and the effects of retinal diseases.